Radiation intensifying screen and radiation receptor and radiation inspection apparatus using the intensifying screen

ABSTRACT

An intensifying screen, comprising a support, a phosphor layer disposed on the support and a protecting film disposed on the phosphor layer. The phosphor layer comprises a first phosphor layer formed on the support side and constituted of particles of the first phosphor having average particle diameter D 1  and range coefficient k, which expresses a particle size distribution, in the range of 1.3 to 1.8, and a second phosphor layer formed on the protective film side and constituted of particles of the second phosphor having average particle diameter D 2  (&gt;D 1 ) and range coefficient k, which expresses a particle size distribution, in the range of 1.5 to 2.0. The ratio (CW 1 :CW 2 ) of coating weight per unit area of the particles of the first phosphor in the first phosphor layer CW 1  and coating weight per unit area of the particles of the second phosphor in the second phosphor layer CW 2  is in the range of from 8:2 to 6:4. According to such intensifying screens, even when phosphors of, for instance, high emission efficiency are employed, while preventing lowering of speed and sharpness from occurring, granularity can be improved.

TECHNICAL FIELD

The present invention relates to intensifying screens employed in X-rayradiography or the like, radiation receptors therewith, and radiationinspection devices therewith.

BACKGROUND ART

In X-ray radiography employed in medical diagnosis and non-destructiveinspection for industrial purpose, in general intensifying screens areused in combination with X-ray film to enhance system sensitivity. Anintensifying screen is generally formed by sequentially forming aphosphor layer and a relatively thin protective film on a supportconsisting of paper or plastic.

In recent years, reduction of subject's exposure to radiation in medicaldiagnosis or the like is strongly demanded. In order to cope with thisdemand, in X-ray radiography, high-speed X-ray films or high-speed X-rayintensifying screens are used to reduce subject's exposure. In order toenhance sensitivity of X-ray film, high speed X-ray films are generallyused. In order to enhance sensitivity of intensifying screens, phosphorsof high emission efficiency are employed.

When X-ray films or intensifying screens are made highly sensitive,there occur the following problems. That is, when the high-speed X-rayfilms are employed, though lowering of sharpness is small, granularityis deteriorated. By contrast, when the high-speed intensifying screensare employed, there also occurs deterioration of granularity.Recognizability of a subject in X-ray radiography involves both ofgranularity and sharpness. Deterioration of granularity deteriorates inparticular the recognizability of subjects of low contrast.

From the above, with an object to improve image quality of intensifyingscreens, various improvements of phosphor layers have been attempted.For instance, when a phosphor layer is produced by the use of a kind ofsettling method named “Ryuen Hou” in Japanese, a phosphor layer of whichparticle size distribution becomes smaller from the protective film sidetoward the support side, a structure in which particle size is gradedcan be obtained (Japanese Patent Publications (KOKOKU) No. Sho 55-33560and No. Hei 1-57758). This kind of structure of phosphor layer canenhance speed and sharpness of intensifying screens.

However, the aforementioned intensifying screens of structure of gradedparticle size distribution are produced by drying solvent while lettingsettle phosphor particles in phosphor slurry by the use of gravity.Accordingly, it takes long time for produce to result in pushing up theproduction cost. In Japanese Patent Publications (KOKOKU) No. Sho55-33560 and No. Hei 1-57758, a structure of multi-layers of phosphorsof different particle sizes is disclosed. These patent publicationsdisclose only examples of the structure of graded particle sizedistribution but does not disclose detailed conditions of each phosphorlayer or the like.

By contrast, Japanese Patent Laid-open Publication (KOKAI) No. Sho58-71500 discloses an intensifying screen in which the surface side of aphosphor layer thereof is constituted of larger phosphor particles of anaverage particle diameter of 7 to 20 μm, and interstices of the largerphosphor particles and support side thereof are constituted of phosphorparticles of an average particle diameter of 4 μm or less. According tosuch an intensifying screen, sensitivity and sharpness can be improvedby some degree. However, granularity can not be sufficiently improved.

In Japanese Patent Laid-open Publication No. (KOKAI) Hei 8-313699, thereis disclosed an intensifying screen having a plurality of phosphorlayers the support side of which layers is composed of phosphorparticles of smaller average particle diameter. Each phosphor layer ofthis intensifying screen, when each average particle diameter ofphosphor particles constituting each phosphor layer is R and particlesize distribution thereof is σ, satisfies a relation of 0<σ/R≦0.5,respectively. Furthermore, in this patent publication, among theplurality of phosphor layers, the phosphor layer of the protective layerside has an average particle diameter of 10 to 20 μm and the phosphorlayer of the support side has an average particle diameter of 1 to 5 μm.

Thus, in an intensifying screen having a plurality of phosphor layers,when particle size diameters of phosphor particles constituting therespective phosphor layers are stipulated similarly, sufficientimprovement of sharpness and granularity is not necessarily obtained. Bythe experiments carried out by the inventors, it has been found thatwhen a plurality of phosphor layers is composed of a plurality ofphosphor particles of different average particle diameters, according toaverage particle diameters of the respective phosphor layers, variouskinds of conditions have to be set.

As mentioned above, high speed intensifying screens due to the use ofphosphors of high emission efficiency can be effective in reduction ofsubject's exposure and in improvement of sharpness, however, cause aproblem of deterioration of granularity. On the contrary, when phosphorsof low emission efficiency are used, the granularity can be improved butthe sharpness deteriorates. Thus, there is a certain degree ofreciprocity between radiographic performance.

As to such problems, existing intensifying screens having a structurecomposed of single phosphor layer can not satisfy both of granularityand sharpness. The intensifying screens having a structure of gradedparticle diameter distribution are relatively satisfactory with respectto speed and sharpness. However, it takes longer time for formation ofphosphor layer to result in pushing-up of manufacturing cost and at thesame time due to fluctuation of manufacturing conditions, largeperformance variation is invited. Further, in the existing intensifyingscreens having a plurality of phosphor layers of different averageparticle diameters, the sharpness and granularity have not beensufficiently improved.

In contrast, radiation is used not only for radiography of medicaldiagnosis but also for treatment of subjects. A device for radiotherapyis one in which a high energy X-ray beam of approximately 4 MV obtainedfrom a linear accelerator called linac is irradiated to an subject tocure. Before beginning treatment with a device for radiotherapy, inorder to confirm reproducibility of a portion being exposed that is setby treatment program, radiography or TV imaging is carried out with thebeam being used for treatment.

However, there is a problem that in the aforementioned high energyX-rays, when an X-ray image is taken with an ordinary intensifyingscreen after transmission of X-rays of a subject, sufficient contrastcan not be obtained. To this end, so far, a fluorometallic screen thatis composed of integration or superposition of an ordinary intensifyingscreen and a metallic plate such as lead alloy foil or copper plate, andmedical X-ray film or industrial X-ray film are combined to employ.Silver halide in film emulsion has the maximum of spectral sensitivityat 45 kV. Accordingly, a high energy X-ray beam of 1 MV or more isabsorbed less to result in poor efficiency. This is the reason why thefluorometallic screen has been employed.

A fluorometallic screen is composed of a phosphor layer of such as CaWO₄in contact with a lead alloy foil, for instance. In such afluorometallic screen, after appropriate absorption of a high energyX-ray beam at the lead alloy foil, a sensitizing effect due to emissionof phosphor, an elimination effect of scattered X-rays due to themetallic foil, a sensitizing effect of phosphor due to secondaryelectrons due to Compton scattering or the like can be obtained.

However, there is a problem from an environment point of view as tohandling of foils of lead alloy. Other than this, plate of heavy metalsuch as tungsten has been taken up. However, tungsten plate is muchexpensive that there is a problem when being put in practice. Incontrast, a fluorometallic screen employing copper plate is small inX-ray absorption, that is, insufficient in absorption of high energyX-rays of 1 MV or more. In addition, existing fluorometallic screens areinsufficient in speed, sharpness or the like, and recognizability ofportions being treated is poor.

An object of the present invention is to provide multi-purposeintensifying screens improved in speed, sharpness, granularity or thelike.

A first more concrete object of the present invention is to provide anintensifying screen employing phosphor of high emission efficiency inwhich, while preventing deterioration of speed and sharpness fromoccurring, granularity is improved and mass-productivity is satisfied.In addition, another object of the present invention is, by employingsuch intensifying screens, to provide a radiation receptor and aradiation inspection device that realize reduction of for instancesubject exposure and improve capability of diagnosis.

A second more concrete object of the present invention is to provide anintensifying screen that has sufficient absorption of high energy X-raysof 1 MV or more, for instance, and is improved in handling performanceduring manufacture and usage, and in speed and sharpness.

DISCLOSURE OF THE INVENTION

In order to look into likelihood of improving performance of anintensifying screen that has a plurality of phosphor layers of differentaverage particle diameters, the present inventors have carried outdetailed experiments concerning particle diameter and particle sizedistribution of phosphor particles constituting the respective phosphorlayers, and packing density of the respective phosphor layers or thelike. As the result of these experiments, it is found that the particlesize distribution and packing amount of each phosphor layer are requiredto be controlled within an appropriate range according to the averageparticle diameter of phosphor particles constituting each layer.

A first intensifying screen of the present invention comprises asupport, a first phosphor layer disposed on the support and constitutedof particles of a first phosphor of which average particle diameter isD₁ and range coefficient k, which expresses particle size distribution,is in the range of 1.3 to 1.8, a second phosphor layer disposed on thefirst phosphor layer and constituted of particles of a second phosphorof which average particle diameter is D₂ that satisfies D₂>D₁ and rangecoefficient k, which expresses particle size distribution, is in therange of 1.5 to 2.0, and a protective layer disposed on the secondphosphor layer.

A second intensifying screen of the present invention comprises asupport, a first phosphor layer disposed on the support and constitutedof particles of a first phosphor having an average particle diameter ofD₁, a second phosphor layer disposed on the first phosphor layer andconstituted of particles of a second phosphor having an average particlediameter of D₂ that satisfies D₂>D₁, and a protective layer disposed onthe second phosphor layer, wherein when a coating weight per unit areaof the particles of the first phosphor in the first phosphor layer isCW₁ and a coating weight per unit area of the particles of the secondphosphor in the second phosphor layer is CW₂, the ratio of the CW₁ andCW₂ (CW₁:CW₂) is in the range of 8:2 to 6:4.

A radiation receptor of the present invention comprises an X-ray film, afront intensifying screen laminated along a surface of the subject sideof the X-ray film and consisting of an intensifying screen of thepresent invention, a back intensifying screen laminated along a surfaceopposite to that of the subject side of the X-ray film and consisting ofan intensifying screen of the present invention, and a cassetteaccommodating a laminate of the front intensifying screen, the X-rayfilm and the back intensifying screen.

A radiation inspection device of the present invention comprises aradiation source, and the aforementioned radiation receptor of thepresent invention that is disposed opposite to the radiation sourcethrough a subject.

Here, it is known that particle size distribution of powder such asphosphor particles can be approximated by lognormal distribution in mostcases. That is, when particle diameter is d, x=log d, an average at thistime is μ, and standard deviation is σ, probability density functionf(x) can be given by the following formula.

f(x)=(1/σ{square root over (2 π)})·(exp [−(x−μ)²/2σ²])

A probability of x being x₀ and less is called a cumulative distributionfunction F (x₀) and is expressed by the following formula.F(x0) = ∫_(−∞)^(x0)f(x)  x

Phosphor particles being measured are put in a dispersion medium such aswater and are dispersed well to measure particle size distribution bythe use of Coulter counter method, micro-track method or the like. Anaverage particle diameter of a phosphor is obtained as a median value ofthis particle size distribution.

FIG. 9 shows an example of a cumulative particle size distribution (interms of weight) of a phosphor employed in intensifying screens of thepresent invention. In the figure, points show actual measurement dataand a curved line shows a theoretical cumulative distribution oflognormal distribution decided so that average value μ and standarddeviation σ thereof meet the measured values. From this example,particle size distribution of phosphor is evident to be expressed wellby the lognormal distribution. The particle size corresponding to 50% ofvertical axis of this cumulative distribution curve is a median value ofthis particle size distribution and denoted as average particle diameterD. Width of particle size distribution can be characterized by rangecoefficient k.

The range coefficient k is defined as follows. When summation of weightof particles in the range of D/k−kD (total weight) is 68.2689% of theweight of whole particles, k is defined as a range coefficient. That is,k is a number of more than 1, the larger the value of k is, the broaderis the particle size distribution, and the closer to 1 the k is, thesharper is the particle size distribution.

The first and second intensifying screens of the present invention havea phosphor layer of two-layer structure. A first phosphor layer thereofis formed on support side and consisting of particles of phosphor ofsmaller particle diameter, and a second phosphor layer thereof is formedon protective film side and consisting of particles of phosphor oflarger particle diameter. In an intensifying screen of phosphor layer oftwo-layer structure, by narrowing the particle size distribution ofphosphor particles of smaller particle diameter and by making relativelybroader the particle size distribution of phosphor particles of largerparticle diameter, sharpness and granularity can be improved. Further,by setting smaller the coating weight per unit area of particles ofphosphor of the first phosphor layer constituted of particles ofphosphor of particle diameter smaller than that of the second phosphorlayer constituted of particles of phosphor of larger particle diameter,sharpness and granularity can be improved.

In the intensifying screen of the present invention, the phosphor layerof two-layer structure can be produced by applying an ordinary producingprocess as identical as the case of the ordinary phosphor layer.Accordingly, in addition to manufacture of intensifying screensthemselves being easy, aimed performance can be obtained withreproducibility. Radiation receptors and radiation inspection devices ofthe present invention, due to adoption of the aforementionedintensifying screens, in particular even when radiography system is madehighly sensitive, can obtain excellent recognizability.

The third intensifying screen of the present invention intends toenhance the contrast of radiographs taken with X-rays of high energysuch as for instance 1 MV or more, and to improve speed, sharpness andgranularity thereof.

That is, a third intensifying screen of the present invention comprisesa support, a phosphor layer disposed on the support, a protective filmdisposed on the phosphor layer, and a powder layer. Here, the powderlayer is disposed between the support and the phosphor layer and isconsisting of at least one kind of particles selected from particles ofsimple metal, particles of alloy consisting mainly of metal andparticles of compound consisting mainly of metal. Here, a thickness ofthe powder layer is in the range of 2 to 40 kg/m² in terms of weight perunit area. As metals to be used for the third intensifying screen, atleast one kind of heavy metals such as W, Mo, Nb and Ta can be cited.

In the third intensifying screen of the present invention, a powderlayer composed of particles of heavy metals such as W, Mo, Nb and Tathat are large in absorption of X-rays of high energy or composed ofparticles consisting mainly of heavy metal is disposed between a supportand a phosphor layer. Such powder layer absorbs the X-rays of highenergy up to an appropriate state corresponding to exposure speed ofX-ray film. Accordingly, excellent contrast that can be applied tomedical diagnosis can be obtained. Further, scattered X-rays can beeffectively absorbed due to the powder layer and a sensitizing effect ofphosphor due to secondary electrons based on Compton scattering can beobtained. As a result of these, speed, sharpness and granularity can beimproved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross section showing an essential structure of oneembodiment of an intensifying screen of the present invention,

FIG. 2 is a diagram showing one example of sharpness performance whenaverage particle diameter D₁ of phosphor particles constituting thefirst phosphor layer is varied in the intensifying screen shown in FIG.1,

FIG. 3 is a diagram showing one example of sharpness performance whenaverage particle diameter D₂ of phosphor particles constituting thesecond phosphor layer is varied in the intensifying screen shown in FIG.1,

FIG. 4 is a diagram showing one example of sharpness performance whenthe ratio of phosphor coating weights of the first phosphor layer andthe second phosphor layer (CW₁:CW₂) is varied in the intensifying screenshown in FIG. 1,

FIG. 5 is a cross section showing a schematic structure of oneembodiment of a radiation receptor of the present invention,

FIG. 6 is a diagram showing one example of sharpness performance whenthe ratio of total coating weights per unit area of phosphor particlesof a front intensifying screen and a back intensifying screen(TCW_(f):TCW_(b)) is varied,

FIG. 7 is a diagram showing diagrammatically a constitution of oneembodiment of a radiation inspection device of the present invention,

FIG. 8 is a cross section showing an essential structure of oneembodiment of another intensifying screen of the present invention,

FIG. 9 is a diagram showing one example of a cumulative particle sizedistribution of phosphor (in terms of weight) employed in anintensifying screen of the present invention.

MODES FOR CARRYING OUT THE INVENTION

In the following, modes for carrying out the present invention will beexplained.

FIG. 1 is a cross section of an essential structure of one embodiment offirst and second intensifying screens of the present invention. In thefigure, reference numeral 1 denotes a support consisting of plastic filmor nonwoven fabric, on one surface of the support 1 a phosphor layer 2being disposed. On the phosphor layer 2, there is disposed a protectivefilm 3 consisting of plastic film or covering film. Of these respectiveelements, an intensifying screen 4 to be used for radiography isconstituted.

A phosphor layer 2 comprises a first phosphor layer 2 a formed on thesupport 1 side and a second phosphor layer 2 b formed on the protectivefilm 3 side. Here, when an average particle diameter of a first phosphorparticles constituting a first phosphor layer 2 a is D₁ and an averageparticle diameter of a second phosphor particles constituting a secondphosphor layer 2 b is D₂, D₁<D₂ is satisfied. That is, on the support 1side, a first phosphor layer 2 a containing phosphor particles ofsmaller particle diameter is disposed, and on the protective film 3side, a second phosphor layer 2 b containing phosphor particles oflarger particle diameter is disposed.

A phosphor layer 2 of two-layer structure consisting of phosphorparticles of different average particle diameters may be formed of CaWO₄phosphor or the like, it is, however, preferable to constituteparticularly of rare earth phosphors such as Gd₂O₂S:Tb, LaOBr:Tb,BaFCl:Eu or the like of high emission efficiency. The first and secondphosphor layers 2 a and 2 b are phosphor layers containing suchparticles of phosphors as described above, respectively.

The intensifying screens 4 involving rare earth phosphors of highemission efficiency are particularly preferable. Even when the rareearth phosphors of high emission efficiency are employed, since thephosphor layer 2 is constituted of two phosphor layers 2 a and 2 b ofdifferent average particle diameters, while preventing deterioration ofspeed and sharpness from occurring, granularity can be improved. Inaddition, the phosphor layers 2 of two-layer structure can be producedsimilarly with the ordinary phosphor layers, resulting in satisfyingmass-productivity.

A first phosphor layer 2 a disposed on a support 1 side is preferable tobe constituted of phosphor particles of smaller particle diameter of anaverage particle diameter D₁ in the range of 1 to 5 μm. In FIG. 2, oneexample of sharpness performance when average particle diameter D₁ ofthe first phosphor particles constituting the first phosphor layer 2 ais varied is shown. By the way, in FIG. 2, Gd₂O₂S:Tb phosphor particlesare employed, average particle diameter D₂ of phosphor particlesconstituting the second phosphor layer 2 b being 9 μm, and rangecoefficient k thereof being 1.6. The ratio (CW₁:CW₂) of coating weightper unit area CW₁ of phosphor particles of smaller particle diameter inthe first phosphor layer 2 a and coating weight per unit area CW₂ ofphosphor particles of larger particle diameter in the second phosphorlayer 2 b is set at 7:3. In FIG. 2, such intensifying screens 4 areemployed as back intensifying screen. Phosphor particles of smallerparticle diameter that are employed here has range coefficient k of 1.5to 1.8.

As obvious from FIG. 2, the smaller the average particle diameter D₁ ofphosphor particles of smaller particle diameter is, the sharper thesharpness becomes. However, when average particle diameter D₁ is lessthan 1 μm, manufacture of phosphor particles itself becomes difficult,and the brightness and formability of the phosphor layer may bedeteriorated. The average particle diameter D₁ of phosphor particles ofsmaller particle diameter constituting the first phosphor layer 2 a ispreferable to be 1 μm or more, accordingly. Further, upon suppressinglowering of the sharpness, the average particle diameter D₁ ispreferable to be set at 5 μm or less, particularly preferable being 3 μmor less. By the way, when the intensifying screen 4 is employed as frontscreen, similar tendency arises.

The second phosphor layer 2 b disposed on the protective film 3 side, inaddition to satisfying D₂>D₁, is preferable to be constituted of largerphosphor particles of average particle diameter D₂ in the range of 5 to20 μm. When the average particle diameter D₂ of phosphor particles isless than 5 μm, even if D₂>D₁ is satisfied, an effect of the secondphosphor layer 2 b employing phosphor particles of larger particle sizecan not be fully obtained.

FIG. 3 shows one example of sharpness performance when average particlediameter D₂ of phosphor particles constituting the second phosphor layer2 b is varied. In FIG. 3, Gd₂O₂S:Tb phosphor particles are employed.Average particle diameter D₁ of phosphor particles constituting thefirst phosphor layer 2 a is 2 μm, range coefficient k is 1.5, and theratio of phosphor coating weights of the first phosphor layer 2 a andthe second phosphor layer 2 b (CW₁:CW₂) is set at 7:3. In FIG. 3, suchintensifying screens 4 are employed as the back screen. Employedphosphor particles of larger particle diameter has range coefficient kin the range of 1.6 to 1.8.

As obvious from FIG. 3, when the average particle diameter D₂ of largerphosphor particles is too large, the sharpness deteriorates largely.Accordingly, the average particle diameter D₂ is preferable to be 20 μmor less, further being preferable to be 10 μm or less. Since thesharpness also deteriorates when the larger phosphor particles has toosmall average particle diameter D₂, the average particle diameter D₂ ispreferable to be 7 μm or more. When the intensifying screen 4 isemployed as the front screen either, similar tendency exists.

Particles of each phosphor constituting the first and second phosphorlayers 2 a and 2 b such as described above have such particle sizedistribution as shown in the following, respectively. That is, thephosphor particles of smaller particle size being employed in the firstphosphor layer 2 a have range coefficient k (k₁), which shows particlesize distribution thereof, in the range of 1.3 to 1.8. By contrast, thephosphor particles of larger particle size being employed in the secondphosphor layer 2 b have range coefficient k (k₂), which shows particlesize distribution thereof, in the range of 1.5 to 2.0. In particular,the range coefficient k₁ of the phosphor particles of smaller particlesize and the range coefficient k₂ of the phosphor particles of largerparticle size are preferable to satisfy k₁<k₂.

Thus, by making narrow the particle size distribution of the phosphorparticles of smaller particle size one side and by making relativelybroad the particle size distribution of the phosphor particles of largerparticle size the other side, sharpness and granularity of the phosphorlayer 2 of two-layer structure can be improved with reproducibility.When phosphor particles (both of smaller size phosphor particles andlarger size phosphor particles) of which range coefficient k deviatesfrom the aforementioned range are employed, improvement effect ofsharpness and granularity due to two-layer structure of the phosphorlayer 2 decreases.

That is, when the range coefficient k₁ of smaller size phosphorparticles constituting the first phosphor layer 2 a is less than 1.3,sharpness and speed are deteriorated largely, and when exceeding 1.8,the sharpness deteriorates. On the other hand, when the rangecoefficient k₂ of larger size phosphor particles constituting the secondphosphor layer 2 b is less than 1.5, the sharpness becomes remarkablylow, and when exceeding 2.0, the sensitivity deteriorates largely. Inaddition, when k₂ is equal with k₁ or smaller than that, the sharpnessdecreases largely.

The range coefficient k₁ of smaller size phosphor particles constitutingthe first phosphor layer 2 a is further preferable to be in the range of1.5 to 1.7. The range coefficient k₂ of larger size phosphor particlesconstituting the second phosphor layer 2 b is further preferable to bein the range of 1.6 to 1.8. By employing the smaller size phosphorparticles and larger size phosphor particles having such rangecoefficients k₁ and k₂, the sharpness and granularity of the phosphorlayer 2 of two-layer structure can be further improved.

Furthermore, the first phosphor layer 2 a and the second phosphor layer2 b, by controlling the ratio of coating weights thereof (CW₁:CW₂)within an appropriate range, can further improve the sharpness andgranularity. In concrete, when the coating weight per unit area ofphosphor particles in the first phosphor layer 2 a is CW₁ and thecoating weight per unit area of phosphor particles in the secondphosphor layer 2 b is CW₂, the ratio (CW₁:CW₂) of these CW₁ and CW₂ ispreferable to be in the range of 8:2 to 6:4.

FIG. 4 shows one example of sharpness performance when the ratio ofcoating weights of the first phosphor layer 2 a and the second phosphorlayer 2 b is varied. In FIG. 4, the ratio of coating weights of phosphoris shown with the ratio (%) of the coating weight of the second phosphorlayer 2 b to the total coating weight of phosphor of the phosphor layer2. In FIG. 4, Gd₂O₂S:Tb phosphor particles are employed. Averageparticle diameter D₁ of phosphor particles constituting the firstphosphor layer 2 a is 2 μm, average particle diameter D₂ of phosphorparticles constituting the second phosphor layer 2 b is 9 μm, and thetotal coating weight per unit area of phosphor particles of the phosphorlayer 2 is 0.60 kg/m². In FIG. 4, such intensifying screen 4 is employedas the front screen.

As obvious from FIG. 4, when the ratio of coating weights of phosphor ofthe first phosphor layer 2 a and the second phosphor layer 2 b (CW₁:CW₂)is in the range of 8:2 to 6:4, excellent sharpness can be obtained. Thesame is with the granularity. When the intensifying screen 4 is employedfor the back screen, similar tendency can be observed.

Thus, by forming a phosphor layer 2 in two-layer structure (D₁<D₂)consisting of the first phosphor layer 2 a and the second phosphor layer2 b of phosphor particles of different average particle sizes, and byfurther setting average particle diameters D₁ and D₂, particle sizedistribution, the ratio of coating weights (CW₁:CW₂) of the firstphosphor layer 2 a and the second phosphor layer 2 b, or the like inappropriate ranges, excellent sensitivity and sharpness can be obtained,and in addition granularity can be improved. The phosphor layers 2 oftwo-layer structure can be manufactured in the identical manner with theordinary phosphor layers. Accordingly, mass-productivity of theintensifying screens 4 can be fully satisfied. In addition, intendedperformance can be obtained with reproducibility.

The intensifying screens of the aforementioned mode can be produced inthe following manner.

That is, smaller size phosphor of which average particle diameter is D₁and range coefficient k₁ is in the range of from 1.3 to 1.8 is mixedwith an appropriate amount of binder. Organic solvent is added theretoto prepare a coating liquid of smaller particle size phosphor ofappropriate viscosity. This coating liquid is used for preparation ofthe first phosphor layer 2 a. On the other hand, larger size phosphor ofwhich average particle diameter is D₂ (>D₁) and range coefficient k₂ isin the range of 1.5 to 2.0 is mixed with an appropriate amount ofbinder. Organic solvent is added thereto to prepare a coating liquid oflarger particle size phosphor of appropriate viscosity. This coatingliquid is used to prepare the second phosphor layer 2 b.

The coating liquid of smaller particle size phosphor being used forpreparation of the first phosphor layer 2 a is coated on a support 1 bythe use of knife coating or roller coating, followed by drying, to forma first phosphor layer 2 a. Next, on the first phosphor layer 2 a, thecoating liquid of larger size phosphor being used for preparation of thesecond phosphor layer 2 b is coated by the use of knife coating orroller coating, followed by drying, to form a second phosphor layer 2 b.

Incidentally, in some cases, there are intensifying screens of astructure in which light reflection layer, light absorption layer, layerof metallic foil or the like is disposed between a support 1 and aphosphor layer 2. In that case, the light reflection layer, lightabsorption layer, layer of metallic foil or the like can be formed inadvance on the support 1, and thereon the phosphor layer 2 needs only beformed.

As binders being employed for preparation of phosphor coating liquid,existing ones such as nitrocellulose, cellulose acetate, ethylcellulose, polyvinyl butyral, flocculate polyester, polyvinyl acetate,vinylidene chloride-vinyl chloride copolymer, vinyl chloride-vinylacetate copolymer, polyalkyl (metha) acrylate, polycarbonate,polyurethane, cellulose acetate butyrate, polyvinyl alcohol or the likecan be cited. As organic solvents, for instance, ethanol, methyl ethylether, butyl acetate, ethyl acetate, ethyl ether, xylene or the like canbe cited. By the way, to the phosphor coating liquid, dispersion agentssuch as phthalic acid, stearic acid or the like and plasticizers such astriphenyl phosphate, diethyl phthalate or the like can be added.

For the support 1, for instance, such resins as cellulose acetate,cellulose propionate, cellulose acetate butyrate, polyesters such aspolyethylene terephthalate, polystyrene, polymethyl methacrylate,polyamide, polyimide, vinyl chloride-vinyl acetate copolymer,polycarbonate or the like can be formed in film to use.

A protective film consisting of transparent resinous film of such aspolyethylene terephthalate, polyethylene, polyvinylidene chloride,polyamide or the like is laminated on the aforementioned phosphor layer2 of two layer structure to form an intended intensifying screen 4.

The protective film 3 may be formed by dissolving resins such ascellulose derivatives such as cellulose acetate, nitrocellulose,cellulose acetate butyrate or the like, polyvinyl chloride, polyvinylacetate, polycarbonate, polyvinyl butyral, polymethyl methacrylate,polyvinyl formal, polyurethane or the like in solvent to form protectivefilm coating liquid of appropriate viscosity, followed by coating anddrying thereof.

The intensifying screen 4 such as described above is used as radiationreceptor 5 such as shown in FIG. 5 in radiography such as X-rayphotography. In the radiation receptor 5 shown in FIG. 5, radiation film6 such as X-ray film is interposed between two sheets of intensifyingscreen 4 (the intensifying screen 4 having the phosphor layer 2 oftwo-layer structure due to the aforementioned mode) and is accommodatedin a cassette 7 in this state.

Among the aforementioned two sheets of intensifying screen 4, one 4 thatis disposed at subject side is so-called front-screen F, and the otherone 4 is so-called back-screen B. The intensifying screens 4 to be usedfor the front intensifying screen F and back intensifying screen B havea basically identical structure as described in the aforementionedembodiment. When the total coating weight per unit area of phosphorparticles in the phosphor layer 2 of two layer structure of the frontintensifying screen F (summation of coating weights of phosphorparticles of the first and second phosphor layers 2 a and 2 b) isTCW_(f) and the total coating weight per unit area of phosphor particlesin the phosphor layer 2 of two layer structure of the back screen B isTCW_(b), the ratio of TCW_(f) and TCW_(b) (TCW_(f):TCW_(b)) ispreferable to be in the range of 3:7 to 4:6.

FIG. 6 shows one example of sharpness performance when the ratio oftotal coating weight per unit area (TCW_(f) ratio) of phosphor particlesof the front screen F and that of the back screen B is varied. By theway, in FIG. 6, Gd₂O₂S:Tb phosphor is employed. The summation of thetotal coating weight per unit area of phosphor particles of the frontscreen F and that of the back screen B is 1.5 kg/m². As obvious fromFIG. 6, when the ratio of the total coating weight per unit area ofphosphor particles of the front screen F and that of the back screen B(TCW_(f):TCW_(b)) is in the range of 3:7 to 4:6, excellent sharpness canbe obtained.

The radiation receptor 5 such as described above is used in a radiationinspection device 8 such as shown in FIG. 7. The radiation inspectiondevice 8 shown in FIG. 7 comprises radiation source 9 and table 11disposed opposite to the radiation source through subject 10 to beinspected such as a patient. The radiation receptor 5 is inserted intothe table 11 from the side of the table 11 to use. At this time, theradiation receptor 5 is inserted so that the front screen F is disposedat the subject 10 side.

The radiation receptor 5 constituted of the intensifying screen 4 of theaforementioned embodiment and the radiation inspection device 8 to beused therewith, even when X-ray exposure to an subject is reducedthrough improvement of system speed, can give excellent recognizability.That is, when used for medical X-ray radiography, for instance, amountof X-ray exposure to a subject can be reduced and excellent diagnosiscan be carried out. When used in industrial nondestructive inspection orthe like, in addition to reduction of an amount of X-rays, inspectionaccuracy can be improved.

Next, concrete embodiments of intensifying screens of the aforementionedmodes and evaluation results thereof will be explained.

EMBODIMENT 1

First, 10 parts by weight of Gd₂O₂S:Tb phosphor powder of which averageparticle diameter is 3 μm and range coefficient k of particle sizedistribution is 1.62 is combined with 1 part by weight of vinylchloride-vinyl acetate copolymer as binder and an appropriate amount ofethyl acetate as organic solvent to prepare a coating liquid of smallerparticle size phosphor. Similarly, 10 parts by weight of Gd₂O₂S:Tbphosphor particles of which average particle diameter is 9 μm and rangecoefficient k of particle size distribution is 1.70 is combined with 1part by weight of vinyl chloride-vinyl acetate copolymer as binder andan appropriate amount of ethyl acetate as organic solvent to prepare acoating liquid of larger size phosphor.

Then, first, the aforementioned coating liquid of smaller size phosphoris coated uniformly on a support by the use of knife coating to be aphosphor coating weight of 0.40 kg/m² after drying, followed by dryingto form a first phosphor layer consisting of smaller particle sizephosphor. The support consists of polyethylene terephthalate film inwhich carbon black is kneaded and of which thickness is 250 μm. Then, onthe first phosphor layer, the coating liquid of larger size phosphor iscoated uniformly by the use of knife coating to be a phosphor coatingweight of 0.20 kg/m² after drying, followed by drying to form a secondphosphor layer consisting of larger size phosphor. Thereafter, on theaforementioned phosphor layer of two layer structure, a protective filmof a thickness of 9 μm is laminated. Thus, first, a front intensifyingscreen is prepared.

On the other hand, the aforementioned coating liquid of smaller sizephosphor is coated uniformly on a support by the use of knife coating tobe a phosphor coating weight of 0.55 kg/m² after drying, followed bydrying to form a first phosphor layer consisting of smaller sizephosphor. The support consists of polyethylene terephthalate film inwhich carbon black is kneaded and of which thickness is 250 μm. Then, onthe first phosphor layer, the coating liquid of larger size phosphor iscoated uniformly by the use of knife coating method to be a phosphorcoating weight of 0.30 kg/m² after drying, followed by drying to form asecond phosphor layer consisting of larger size phosphor. Thereafter, onthe aforementioned phosphor layer of two layer structure, a protectivefilm of a thickness of 9 μm is laminated. Thus, a back intensifyingscreen is prepared.

In the intensifying screens for the front and back intensifying screens,the ratio of coating weights CW₁:CW₂ of the front intensifying screen is6.7:3.3 and for the back intensifying screen, CW₁:CW₂ is 6.5:3.5. Inaddition, the ratio of the total phosphor coating weights of the frontscreen and back screen TCW_(f):TCW_(b) is 4.1:5.9. Such front and backintensifying screens are provided for performance evaluation.

Comparative Example 1

10 parts by weight of Gd₂O₂S:Tb phosphor powder of which averageparticle diameter is 6.5 μm and range coefficient k of particle sizedistribution is 1.55 is combined with 1 part by weight of vinylchloride-vinyl acetate copolymer as binder and an appropriate amount ofethyl acetate as organic solvent to prepare a coating liquid ofphosphor. The aforementioned coating liquid of phosphor is coateduniformly on a support by the use of knife coating to be a phosphorcoating weight of 0.45 kg/m² after drying, followed by drying to form aphosphor layer. The support consists of polyethylene terephthalate filmin which titanium white is kneaded and of which thickness is 250 μm.Thereafter, on the phosphor layer of one layer structure, a protectivefilm of a thickness of 9 μm is laminated. Thus, a front intensifyingscreen is prepared.

On the other hand, on a support consisting of polyethylene terephthalatefilm in which titanium white is kneaded and of which thickness is 250μm, the aforementioned phosphor coating liquid is coated uniformly bythe use of knife coating to be phosphor coating weight of 0.55 kg/m²after drying, followed by drying to form a phosphor layer. Thereafter,on the phosphor layer of one layer structure, a protective film of athickness of 9 μm is laminated. Thus, a back intensifying screen isprepared. These front and back intensifying screens are provided for theperformance evaluation that will be described later.

Comparative Example 2

In the aforementioned embodiment 1, for the smaller size phosphor,Gd₂O₂S:Tb phosphor powder of which average particle diameter is 3 μm andrange coefficient k of particle size distribution is 1.13 is employed,and for the larger size phosphor, Gd₂O₂S:Tb phosphor powder of whichaverage particle diameter is 9 μm and range coefficient k of particlesize distribution is 1.40 is employed. Except for the above, in theidentical way with the embodiment 1, the front and back intensifyingscreens are prepared. Such front and back intensifying screens areprovided for the performance evaluation that will be described later.

EMBODIMENT 2

First, 10 parts by weight of Gd₂O₂S:Tb phosphor powder of which averageparticle diameter is 3 μm and range coefficient k of particle sizedistribution is 1.62 is combined with 1 part by weight of vinylchloride-vinyl acetate copolymer as binder and an appropriate amount ofethyl acetate as organic solvent to prepare a coating liquid of smallersize phosphor. Similarly, 10 parts by weight of Gd₂O₂S:Tb phosphorpowder of which average particle diameter is 9 μm and range coefficientk of particle size distribution is 1.70 is combined with 1 part byweight of vinyl chloride-vinyl acetate copolymer as binder and anappropriate amount of ethyl acetate as organic solvent to prepare acoating liquid of larger size phosphor.

Then, first, the aforementioned coating liquid of smaller size phosphoris coated uniformly on a support by the use of knife coating to be aphosphor coating weight of 0.40 kg/m² after drying, followed by dryingto form a first phosphor layer consisting of smaller size phosphor. Thesupport consists of polyethylene terephthalate film in which titaniumwhite is kneaded and of which thickness is 250 μm. Then, on the firstphosphor layer, the coating liquid of larger size phosphor is coateduniformly by the use of knife coating to be a phosphor coating weight of0.20 kg/m² after drying, followed by drying to form a second phosphorlayer consisting of larger size phosphor. Thereafter, on theaforementioned phosphor layer of two layer structure, a protective filmof a thickness of 9 μm is laminated. Thus, first, a front intensifyingscreen is prepared.

On the other hand, the aforementioned coating liquid of smaller sizephosphor is coated uniformly on a support by the use of knife coating tobe a phosphor coating weight of 0.70 kg/m² after drying, followed bydrying to form a first phosphor layer consisting of smaller sizephosphor. The support consists of polyethylene terephthalate film inwhich titanium white is kneaded and of which thickness is 250 μm. Then,on the first phosphor layer, the coating liquid of larger size phosphoris coated uniformly by the use of knife coating to be a phosphor coatingweight of 0.35 kg/m² after drying, followed by drying to form a secondphosphor layer consisting of larger size phosphor. Thereafter, on theaforementioned phosphor layer of two layer structure, a protective filmof a thickness of 9 μm is laminated. Thus, a back intensifying screen isprepared.

In the front and back intensifying screens, the ratio of coating weightsCW₁:CW₂ of the front intensifying screen is 6.7:3.3 and of the backintensifying screen, CW₁:CW₂ is 6.7:3.3. In addition, the ratio of thetotal phosphor coating weights of the front screen and back screenTCW_(f):TCW_(b) is 3.6:6.4. Such front and back intensifying screens areprovided for performance evaluation that will be described later.

Comparative Example 3

10 parts by weight of Gd₂O₂S:Tb phosphor powder of which averageparticle diameter is 10.8 μm and range coefficient k of particle sizedistribution is 1.60 is combined with 1 part by weight of vinylchloride-vinyl acetate copolymer as binder and an appropriate amount ofethyl acetate as organic solvent to prepare a coating liquid ofphosphor. The aforementioned coating liquid of phosphor is coateduniformly on a support by the use of knife coating to be a phosphorcoating weight of 0.55 kg/m² after drying, followed by drying to form aphosphor layer. The support consists of polyethylene terephthalate filmin which titanium white is kneaded and of which thickness is 250 μm.Thereafter, on the phosphor layer of one layer structure, a protectivefilm of a thickness of 9 μm is laminated. Thus, a front intensifyingscreen is prepared.

On the other hand, on a support consisting of polyethylene terephthalatefilm in which titanium white is kneaded and of which thickness is 250μm, the aforementioned coating liquid of phosphor is coated uniformly bythe use of knife coating to be a phosphor coating weight of 1.15 kg/m²after drying, followed by drying to form a phosphor layer. Thereafter,on the phosphor layer of one layer structure, a protective film of athickness of 9 μm is laminated. Thus, a front intensifying screen isprepared. These front and back intensifying screens are provided for theperformance evaluation that will be described later.

Comparative Example 4

In the aforementioned embodiment 2, for the smaller particle sizephosphor, Gd₂O₂S:Tb phosphor powder of which average particle diameteris 3 μm and range coefficient k of particle size distribution is 1.95 isemployed, and for the larger size phosphor, Gd₂O₂S:Tb phosphor powder ofwhich average particle diameter is 9 μm and range coefficient k ofparticle size distribution is 2.10 is employed. Except for the above, inthe identical way with the embodiment 2, front and back intensifyingscreens are prepared. Such intensifying screens for the uses of frontand back screens are provided for the performance evaluation that willbe described later.

EMBODIMENT 3

First, 10 parts by weight of CaWO₄ phosphor powder of which averageparticle diameter is 3.5 μm and range coefficient k of particle sizedistribution is 1.53 is combined with 1 part by weight of vinylchloride-vinyl acetate copolymer as binder and an appropriate amount ofethyl acetate as organic solvent to prepare a coating liquid of smallersize phosphor. Similarly, 10 parts by weight of CaWO₄ phosphor powder ofwhich average particle diameter is 15.7 μm and range coefficient k ofparticle size distribution is 1.65 is combined with 1 part by weight ofvinyl chloride-vinyl acetate copolymer as binder and an appropriateamount of ethyl acetate as organic solvent to prepare a coating liquidof larger size phosphor.

Then, first, the aforementioned coating liquid of smaller size phosphoris coated uniformly on a support by the use of knife coating to be aphosphor coating weight of 0.30 kg/m² after drying, followed by dryingto form a first phosphor layer consisting of smaller size phosphor. Thesupport consists of polyethylene terephthalate film in which carbonblack is kneaded and of which thickness is 250 μm. Then, on the firstphosphor layer, the coating liquid of larger size phosphor is coateduniformly by the use of knife coating to be a phosphor coating weight of0.20 kg/m² after drying, followed by drying to form a second phosphorlayer consisting of larger size phosphor. Thereafter, on theaforementioned phosphor layer of two layer structure, a protective filmof a thickness of 9 μm is laminated. Thus, first, a front intensifyingscreen is prepared.

On the other hand, the aforementioned coating liquid of smaller sizephosphor is coated uniformly on a support by the use of knife coating tobe a phosphor coating weight of 0.50 kg/m² after drying, followed bydrying to form a first phosphor layer consisting of smaller sizephosphor. The support consists of polyethylene terephthalate film inwhich carbon black is kneaded and of which thickness is 250 μm. Then, onthe first phosphor layer, the coating liquid of larger size phosphor iscoated uniformly by the use of knife coating to be a phosphor coatingweight of 0.30 kg/m² after drying, followed by drying to form a secondphosphor layer consisting of larger size phosphor. Thereafter, on theaforementioned phosphor layer of two layer structure, a protective filmof a thickness of 9 μm is laminated. Thus, a back intensifying isprepared.

In the front and back intensifying screens, the ratio of phosphorcoating weights CW₁:CW₂ of the front intensifying screen is 6:4 and ofthe back intensifying screen, CW₁:CW₂ is 6.3:3.7. In addition, the ratioof the total phosphor coating weights of the front screen and backscreen TCW_(f):TCW_(b) is 3.8:6.2. Such front and back intensifyingscreens are provided for performance evaluation that will be describedlater.

Comparative Example 5

10 parts by weight of CaWO₄ phosphor powder of which average particlediameter is 10.0 μm and range coefficient k of particle sizedistribution is 1.40 is combined with 1 part by weight of vinylchloride-vinyl acetate copolymer as binder and an appropriate amount ofethyl acetate as organic solvent to prepare a coating liquid ofphosphor. The aforementioned coating liquid of phosphor is coateduniformly on a support by the use of knife coating to be a phosphorcoating weight of 0.60 kg/m² after drying, followed by drying to form aphosphor layer. The support consists of polyethylene terephthalate filmin which titanium white is kneaded and of which thickness is 250 μm.Thereafter, on the phosphor layer of one layer structure, a protectivefilm of a thickness of 9 μm is laminated. Thus, a front intensifying isprepared.

On the other hand, on a support consisting of polyethylene terephthalatefilm in which titanium white is kneaded and of which thickness is 250μm, the aforementioned coating liquid of phosphor is coated uniformly bythe use of knife coating to be a phosphor coating weight of 0.90 kg/m²after drying, followed by drying to form a phosphor layer. Thereafter,on the phosphor layer of one layer structure, a protective film of athickness of 9 μm is laminated. Thus, a front intensifying screen isprepared. These front and back intensifying screens are provided for theperformance evaluation that will be described later.

Comparative Example 6

In the aforementioned embodiment 3, for the smaller size phosphor, CaWO₄phosphor powder of which average particle diameter is 3.5 μm and rangecoefficient k of particle size distribution is 1.20 is employed, and forthe larger size phosphor, CaWO₄ phosphor powder of which averageparticle diameter is 15.7 μm and range coefficient k of particle sizedistribution is 1.45 is employed. Except for the above, in the identicalway with the embodiment 3, front and back intensifying screens areprepared. Such front and back intensifying screens are provided for theperformance evaluation that will be described later.

EMBODIMENT 4

First, 10 parts by weight of BaFCl:Eu phosphor powder of which averageparticle diameter is 3.8 μm and range coefficient k of particle sizedistribution is 1.58 is combined with 1 part by weight of vinylchloride-vinyl acetate copolymer as binder and an appropriate amount ofethyl acetate as organic solvent to prepare a coating liquid of smallersize phosphor. Similarly, 10 parts by weight of BaFCl:Eu phosphor powderof which average particle diameter is 8.5 μm and range coefficient k ofparticle size distribution is 1.65 is combined with 1 part by weight ofvinyl chloride-vinyl acetate copolymer as binder and an appropriateamount of ethyl acetate as organic solvent to prepare a coating liquidof larger size phosphor.

Then, first, the aforementioned coating liquid of smaller size phosphoris coated uniformly on a support by the use of knife coating to be aphosphor coating weight of 0.30 kg/m² after drying, followed by dryingto form a first phosphor layer consisting of smaller size phosphor. Thesupport consists of polyethylene terephthalate film in which titaniumwhite is kneaded and of which thickness is 250 μm. Then, on the firstphosphor layer, the coating liquid of larger size phosphor is coateduniformly by the use of knife coating to be a phosphor coating weight of0.20 kg/m² after drying, followed by drying to form a second phosphorlayer consisting of larger size phosphor. Thereafter, on theaforementioned phosphor layer of two layer structure, a protective filmof a thickness of 9 μm is laminated. Thus, front and back intensifyingscreens are prepared.

In the front and back intensifying screens, the ratio of coating weightsCW₁:CW₂ of the front intensifying screen and back intensifying screen is6:4. In addition, the ratio of the total phosphor coating weights of thefront screen and back screen TCW_(f):TCW_(b) is 5:5. Such front and backintensifying screens are provided for performance evaluation that willbe described later.

Comparative Example 7

10 parts by weight of BaFCl:Eu phosphor powder of which average particlediameter is 4.5 μm and range coefficient k of particle size distributionis 1.50 is combined with 1 part by weight of vinyl chloride-vinylacetate copolymer as binder and an appropriate amount of ethyl acetateas organic solvent to prepare a coating liquid of phosphor. The coatingliquid of phosphor is coated uniformly on a support by the use of knifecoating to be a phosphor coating weight of 0.50 kg/m² after drying,followed by drying to form a phosphor layer. The support consists ofpolyethylene terephthalate film in which titanium white is kneaded andof which thickness is 250 μm. Thereafter, on the phosphor layer of onelayer structure, a protective film of a thickness of 9 μm is laminated.Thus, front and back intensifying screens are prepared. These front andback intensifying screens are provided for performance evaluation thatwill be described later.

Comparative Example 8

In the aforementioned embodiment 4, for the smaller size phosphor,BaFCl:Eu phosphor powder of which average particle diameter is 3.8 μmand range coefficient k of particle size distribution is 1.85 isemployed, and for the larger size phosphor, BaFCl:Eu phosphor powder ofwhich average particle diameter is 8.5 μm and range coefficient k ofparticle size distribution is 1.40 is employed. Except for the above, inthe identical way with the embodiment 4, front and back intensifyingscreens are prepared. Such front and back intensifying screens areprovided for the performance evaluation that will be described later.

The respective intensifying screen pairs (pair of a front intensifyingscreen and a back intensifying screen) due to the aforementionedEmbodiments 1 and 2, and Comparative Examples 1, 2, 3 and 4 areevaluated of their sensitivity, sharpness and granularity withortho-type X-ray film (product name of Konica: SR-G). The respectiveintensifying screen pairs due to the aforementioned Embodiments 3 and 4,and Comparative Examples 5, 6, 7 and 8 are evaluated of theirsensitivity, sharpness and granularity with regular-type X-ray film(product name of Konica: New-A). The results thereof are shown in Table1.

By the way, photographic performance of the aforementioned intensifyingscreen pairs is evaluated of sensitivity, sharpness and granularity withX-rays of tube-voltage of 120 kV after transmission of a water phantomof a thickness of 100 mm. The sensitivity is expressed in terms ofrelative value with each value of comparative example 1, 3, 5 and 7 as100, respectively. The sharpness, after evaluating the respective MTFsat a spatial frequency of 2 lines/mm, is expressed in terms of relativevalues with each value of comparative examples 1, 3, 5 and 7 as 100,respectively. The granularity is expressed as relative RMS value at aspatial frequency of 3.12 line/mm under photographic density of 1.0.

TABLE 1 Thickness of protective Sensi- Sharp- Kind of Phosphor layerstructure of film tivity ness Relative phosphor front F and back B (μm)Support (%) (%) RMS (%) Embodiment Gd₂O₂S:Tb F:3 μm (k = 1.62), 0.40kg/m² 9 carbon black 120 120 130 1 /9 μm (k = 1.70), 0.20 kg/m² B:3 μm(k = 1.62), 0.55 kg/m² 9 carbon black /9 μm (k = 1.70), 0.30 kg/m²Comparative Gd₂O₂S:Tb F:6.5 μm, 0.50 kg/m² 9 titanium 100 100 100example 1 white B:6.5 μm, 0.55 kg/m² 9 titanium white ComparativeGd₂O₂S:Tb F:3 μm (k = 1.13), 0.40 kg/m² 9 carbon black 121 108 125example 2 /9 μm (k = 1.40), 0.20 kg/m² B:3 μm (k = 1.13), 0.55 kg/m² 9carbon black /9 μm (k = 1.40), 0.30 kg/m² Embodiment Gd₂O₂S:Tb F:3 μm (k= 1.62), 0.40 kg/m² 9 titanium 90 140 110 2 /9 μm (k = 1.70), 0.20 kg/m²white B:3 μm (k = 1.62), 0.70 kg/m² 9 titanium /9 μm (k = 1.70), 0.35kg/m² white Comparative Gd₂O₂S:Tb F:10.8 μm, 0.55 kg/m² 9 titanium 100100 100 example 3 white F:10.8 μm, 1.15 kg/m² 9 titanium whiteComparative Gd₂O₂S:Tb F:3 μm (k = 1.95), 0.40 kg/m² 9 titanium 85 120112 example 4 /9 μm (k = 2.10), 0.20 kg/m² white B:3 μm (k = 1.95), 0.70kg/m² 9 titanium /9 μm (k = 2.10), 0.35 kg/m² white

TABLE 2 Thickness of Sensi- Kind of Phosphor layer structure protectivetivity Sharpness Relative phosphor of front F and back B film (μm)Support (%) (%) RMS (%) Embodiment 3 CaWO₄ F:3.5 μm (k = 1.53), 0.30 9carbon black 100 115 110 kg/m² /15.7 μm (k = 1.65), 0.20 kg/m² B:3.5 μm(k = 1.53), 0.50 9 carbon black kg/m² /15.7 μm (k = 1.65), 0.30 kg/m²Comparative CaWO₄ F:10.0 μm, 0.60 kg/m² 9 carbon black 100 100 100example 5 B:10.0 μm, 0.90 kg/m² 9 titanium white Comparative CaWO₄ F:3.5μm (k = 1.20), 0.30 9 carbon black 100 106 108 example 6 kg/m² /15.7 μm(k = 1.45), 0.20 kg/m² B:3.5 μm (k = 1.20), 0.50 9 carbon black kg/m²/15.7 μm (k = 1.45), 0.30 kg/m² Embodiment 4 BaFCl:Eu F:3.8 μm (k =1.58), 0.30 9 titanium white 110 110 120 kg/m² /8.5 μm (k = 1.65), 0.20kg/m² B:3.8 μm (k = 1.58), 0.30 9 titanium white kg/m² /8.5 μm (k =1.65), 0.20 kg/m² Comparative BaFCl:Eu F:4.5 μm, 0.50 kg/m² 9 titaniumwhite 100 100 100 example 7 B:4.5 μm, 0.50 kg/m² 9 carbon blackComparative BaFCl:Eu F:3.8 μm (k = 1.85), 0.30 9 titanium white 111 103120 example 8 kg/m² /8.5 μm (k = 1.40), 0.20 kg/m² B:3.8 μm (k = 1.85),0.30 9 titanium white kg/m² /8.5 μm (k = 1.40), 0.20 kg/m²

As obvious from Tables 1 and 2, all of the respective intensifyingscreen pairs (pair of a front intensifying screen and a backintensifying screen) due to Embodiments 1 through 4, compared withintensifying screen pairs of single layer structure, are improved intheir granularity. In addition to this improvement, lowering ofsensitivity or sharpness is small or improved.

Next, another embodiments for implementing intensifying screens of thepresent invention will be described.

FIG. 8 is a cross section showing a structure of one embodiment of athird intensifying screen of the present invention. In the same figure,reference numeral 21 denotes a support consisting of plastic film ornonwoven fabric. On one surface the support 21, there is disposed apowder layer 22. The powder layer consists of at least one kind ofparticles selected from particles of simple metal, particles of alloyconsisting mainly of metal and particles of compound consisting mainlyof metal and has a thickness of 2 to 40 kg/m² in terms of weight perunit area.

The powder layer 22, as will be explained in detail later, is disposedso as to absorb X-rays of high energy to be the intensity of the X-raysof high energy appropriate for the sensitivity of X-ray film. Further,the powder layer 22, due to an elimination effect of scattered X-raysand a sensitizing effect of phosphor due to secondary electrons based onCompton scattering, improves sensitivity, sharpness and granularity.Upon obtaining such effects, as the metal constituting the powder layer22, at least one kind of heavy metal selected from W, Mo, Nb and Ta ispreferable.

On the powder layer 22, there is disposed a phosphor layer 23. For thephosphors constituting the phosphor layer 23, generally used CaWO₄ maybe employed and also rare earth phosphors of high emission efficiencysuch as BaFCl:Eu, Gd₂O₂S:Tb, LaOBr:Tb or the like may be used. Thephosphor layer 23 contains particles of such phosphors.

On the phosphor layer 23, a protective film 24 consisting of plasticfilm or plastic cover film is disposed. With these elements, an X-rayintensifying screen 25 being used in high energy X-ray radiography of 1MV or more is constituted. The X-ray intensifying screen 25 of thisembodiment is suitable for one that is used to confirm an irradiationarea prior to treatment with X-rays of high energy for treatment such asapproximately 4 MV that is obtained by a linear accelerator called aslinac.

For particles constituting the aforementioned powder layer 22, at leastone kind of particles selected from simple particles of heavy metals, inparticular of W, Mo, Nb, Ta or the like, alloy particles consistingmainly of these metals, and compound particles consisting mainly ofthese metals can be employed.

In concrete, simple particles of metals such as W particles, Moparticles, Nb particles and Ta particles, alloy particles consistingmainly of heavy metals such as W—Re alloy particles, W—Mo alloyparticles, W—Nb alloy particles, W—Ta alloy particles, Mo—Nb alloyparticles, Mo—Ta alloy particles and Nb—Ta alloy particles, and compoundparticles consisting mainly of heavy metals such as particles oftungsten carbide (WC), particles of tungsten oxides (such as WO₃ or thelike), particles of molybdenum oxides (such as MoO₃ or the like),particles of tungsten carbide (MoC), particles of niobium carbide (Nb—C)and particles of tantalum carbide (Ta—C) can be employed. Compoundsconsisting mainly of refractory metals, without restricting to oxidesand carbides, can be various kinds of compounds such as intermetalliccompounds or the like, and are not limited to particular types ofcompounds.

However, when particles of alloys or compounds consisting mainly ofheavy metals are employed, alloys or compounds of which amount of heavymetal is 60% or more by weight in these particles are preferable. Whenthe heavy metal is contained less than 60% by weight in the particles,there is a danger that an absorption effect of X-rays of high energy cannot be obtained fully. In other words, alloys or compounds of whichheavy metal is 60% or more by weight can give an effect similar to thatobtained by simple particles of heavy metals.

Heavy metals such as W, Mo, Nb and Ta that are main constituents of thepowder layer 22 can largely absorb X-rays of high energy such asdescribed above. Accordingly, when the X-ray intensifying screen 25 ofthis embodiment is employed for radiography as a preparatory inspectionmeans of X-ray treatment with X-rays of high energy, the high energyX-rays irradiated from the support 21 side, before reaching the phosphorlayer 23, is absorbed to the value appropriate for exposure sensitivityof such as X-ray film.

In addition, even if the X-rays converted to an appropriate energy stateby going through the powder layer 22 are scattered by the phosphor layer23 or the protective film 24, the scattered X-rays can be effectivelyabsorbed by the powder layer 22. Thus, by effectively absorbing thescattered X-rays by the powder layer 22, the scattered X-rays can bemade less probable in reentering into the phosphor layer 23, thegranularity and sharpness can be improved accordingly. Furthermore,since the powder layer 22 consisting mainly of heavy metals such as W orthe like has a sensitizing effect of phosphor due to secondary electronsbased on Compton scattering, the sensitivity and sharpness can befurther improved.

The thickness of the powder layer 22 constituted mainly of heavy metalsis in the range of 2 to 40 kg/m² in terms of weight per unit area. Whenthe thickness of the powder layer 22 is less than 2 kg/m² in terms ofweight per unit area, the X-rays of high energy can not be effectivelyabsorbed, resulting in exposure of less contrast of X-ray film. On theother hand, when the thickness of the powder layer 22 exceeds 40 kg/m²in terms of weight per unit area, absorption of the X-rays becomes toolarge, resulting in lowering of sensitivity. The thickness of the powderlayer 22 is preferable to be in the range of 5 to 30 kg/m² in terms ofweight per unit area.

The powder layer 22 can be formed in the similar manner with thephosphor layer 23. That is, particles selected from for instance simpleparticles of W, alloy particles consisting mainly of W or compoundparticles consisting mainly of W are mixed with adequate amount ofbinder and organic solvent is added thereto to prepare a powder coatingliquid of appropriate viscosity. This powder coating liquid is coated ona support 21 by the use of knife coating or roller coating and dried toresult in a desired powder layer 22. According to such coating methods,the powder layer 22 having the aforementioned thickness can be obtainedeasily and less expensively.

Thus, in the intensifying screen 25, X-rays of high energy are absorbedby the powder layer 22 consisting mainly of heavy metals to be a stateadequate for radiography and, further an absorption effect of scatteredX-rays and a sensitizing effect of phosphor due to secondary electronsbased on Compton scattering can be obtained. Accordingly, in radiographyemploying high energy X-rays, in addition to excellent contrast andsensitivity, granularity and sharpness can be improved.

Improvement effects of sensitivity, granularity and sharpness can beobtained with a simple structure in which powder layer 22 is disposedbetween support 21 and phosphor layer 23. As a result of this,intensifying screens 25 that can cope with the X-rays of high energysuch as 1 MV or more and can improve the granularity and sharpness canbe produced with ease and less expensively. In addition, there is nohandling problem as existing fluorometallic screens cause and they areadvantageous from the viewpoint of cost.

According to the intensifying screens 25 of this embodiment, even whenradiographs are taken with X-rays of high energy for treatment such asapproximately 4 MV, due to existence of the powder layer 22, theexcellent contrast can be obtained. In addition, since excellentsensitivity, granularity and sharpness can be obtained, when theintensifying screen is employed for radiography (medical radiography) aspreparatory inspection means of X-ray treatment employing X-rays of highenergy, reproducibility of an irradiation field set by a treatmentprogram can be clearly confirmed. That is, excellent recognizability ofportions to be treated can be obtained.

Intensifying screens 25 of the aforementioned embodiment can be producedby the following way, for instance.

That is, at least one kind of particles selected from simple particlesof metals, alloy particles having heavy metals as main constituent andcompound particles having heavy metals as main constituent are mixedwith an appropriate amount of binder, followed by addition of organicsolvent to result in a powder coating liquid of appropriate viscosity.This powder coating liquid is coated on a support 21 by the use of knifecoating or roller coating and is dried to result in a powder layer 22consisting mainly of heavy metals.

As binders being employed for preparation of powder coating liquid,nitrocellulose, cellulose acetate, ethyl cellulose, polyvinyl butyral,flocculate polyester, polyvinyl acetate, vinylidene chloride-vinylchloride copolymer, vinyl chloride-vinyl acetate copolymer, polyalkyl(metha) acrylate, polycarbonate, polyurethane, cellulose acetatebutyrate, polyvinyl alcohol or the like can be employed. As organicsolvents, for instance, ethanol, methyl ethyl ether, butyl acetate,ethyl acetate, ethyl ether, xylene or the like can be cited. By the way,to the powder coating liquid, dispersion agent such as phthalic acid,stearic acid or the like and plasticizer such as triphenyl phosphate,diethyl phthalate or the like can be added.

For the support 21, for instance, such resins as cellulose acetate,cellulose propionate, cellulose acetate butyrate, polyesters such aspolyethylene terephthalate, polystyrene, polymethyl methacrylate,polyamide, polyimide, vinyl chloride-vinyl acetate copolymer,polycarbonate or the like can be formed in film to use.

On the other hand, phosphor is mixed with an appropriate amount ofbinder, followed by addition of organic solvent to prepare a phosphorcoating liquid of appropriate viscosity. This phosphor coating liquid iscoated on a protective layer 24 by the use of knife coating or rollercoating and dried to form a phosphor layer 23. Binders or organicsolvents being used for preparation of the phosphor coating liquid canbe similar ones employed for preparation of the powder coating liquid.For protective film 24, such transparent resinous films as polyethyleneterephthalate, polyethylene, polyvinylidene chloride and polyamide canbe employed. As demands arise, dispersion agents such as phthalic acid,stearic acid or the like or plasticizer such as triphenyl phosphate,diethyl phthalate or the like can be added to phosphor coating liquid.

By laminating a support 21 thereon the powder layer 22 containing theaforementioned heavy metals such as W or Mo is formed and a protectivefilm thereon a phosphor layer 23 is formed, an intended X-rayintensifying screen (radiation intensifying screen) 25 can be obtained.

By the way, by coating the phosphor coating liquid directly on thepowder layer 22 and drying, followed by laminating thereon a filmyprotective film 4 or by coating thereon a protective film coating liquidthat is adjusted to an appropriate viscosity by dissolving various kindsof resins in solvent, followed by drying, an X-ray intensifying screen25 can be produced.

The X-ray intensifying screens 25 can be produced with other method thanthat described above. That is, a protective film 24 is formed in advanceon a flat plate and thereon a phosphor layer 23 and a powder layer 22are formed sequentially. Thereafter, together with the protective filmthey are peeled off the plate and on the powder layer 22 thereof asupport 21 is laminated.

Next, concrete embodiments of the radiation intensifying screens (X-rayintensifying screen 25) of the aforementioned implementing modes andevaluation results thereof will be described.

EMBODIMENT 5

First, 10 parts by weight of Gd₂O₂S:Tb phosphor powder of which averageparticle diameter is 6.0 μm is combined with 1 part by weight of vinylchloride-vinyl acetate copolymer as binder and an appropriate amount ofethyl acetate as organic solvent to prepare a phosphor coating liquid.The phosphor coating liquid is coated uniformly on a protective filmconsisting of polyethylene terephthalate film of a thickness of 9 μm bythe use of knife coating to be a phosphor coating weight of 1.20 kg/m²after drying, followed by drying to form a phosphor layer.

On the other hand, 1 part by weight of particles of W metal of anaverage particle diameter of 3.0 μm is combined with 1 part by weight ofvinyl chloride-vinyl acetate copolymer as binder and an appropriateamount of ethyl acetate as organic solvent to prepare a W particlecoating liquid. The W particle coating liquid is coated uniformly on asupport by the use of knife coating to be a coating weight of Wparticles of 10 kg/m² ₁ followed by drying to form a W powder layer(powder layer). The support consists of polyethylene terephthalate filmof which thickness is 250 μm and in which carbon black is kneaded.

Thereafter, the protective film thereon the phosphor layer is formed andthe support thereon the W powder layer is formed are laminated so thatthe phosphor layer face the W powder layer, resulting in an intendedX-ray intensifying screen. This X-ray intensifying screen is providedfor performance evaluation to be described later.

EMBODIMENTS 6 AND 7

As constituent particles of powder layer, WC (tungsten carbide)particles of an average particle diameter of 3.5 μm (Embodiment 6) andW—Re alloy particles (Embodiment 7) of an average particle diameter of4.0 μm are coated to be coating weights of 15 kg/m² (Embodiment 6) and16 kg/m² (Embodiment 7), respectively. Except for the above, asidentical with Embodiment 5, X-ray intensifying screens are produced,respectively. These X-ray intensifying screens are provided forperformance evaluation to be described later.

EMBODIMENT 8 TO 10

As constituent particles of powder layer, Mo particles (Embodiment 8) ofan average particle diameter of 5 μm, Nb particles (Embodiment 9) of anaverage particle diameter of 8 μm and Ta particles (Embodiment 10) of anaverage particle diameter of 7 μm are coated to be coating weights of 19kg/m² (Embodiment 8), 18 kg/m² (Embodiment 9), and 11 kg/m² (Embodiment10), respectively. Except for the above, as identical with Embodiment 5,X-ray intensifying screens are produced, respectively. These X-rayintensifying screens are provided for performance evaluation to bedescribed later.

Comparative Example 9

In the place of the powder layer in Embodiment 5, a lead foil of athickness of 0.5 mm is employed. In the identical manner with embodiment1 except for the above, X-ray intensifying screens are prepared. TheseX-ray intensifying screens are supplied for performance evaluation to bedescribed later.

The respective X-ray intensifying screens of the aforementionedEmbodiments 5 through 10 and Comparative Example 9 are evaluated ofsensitivity and sharpness with ortho-type X-ray film (Fuji Photo-FilmCo: Super HR-S) when X-rays of energy of 4 MV are irradiated. Theresults are shown in Table 3. By the way, each of photographicsensitivity of intensifying screens is shown as relative value with thevalue of comparative example as 100. The sharpness, by evaluating MTFsat spatial frequency of 2 lines/mm, is shown as relative values withthat of intensifying screen of comparative example 9 as 100.

TABLE 3 Powder layer Average Phosphor Layer Consti- Particle CoatingCoating Sensi- Sharp- tuent Diameter Weight Weight tivity ness Particle(μm) (kg/m²) Phosphor (kg/m²) (%) (%) Embodiment 5 W 3.0 10 Gd₂O₂S:Tb1.20 100 110 Embodiment 6 WC 3.5 15 Gd₂O₂S:Tb 1.20 100 108 Embodiment 7W-Re 4.0 16 Gd₂O₂S:Tb 1.20 100 105 Embodiment 8 Mo 5.0 19 Gd₂O₂S:Tb 1.20101 115 Embodiment 9 Nb 8.0 18 Gd₂O₂S:Tb 1.20 98 109 Embodiment 10 Ta7.0 11 Gd₂O₂S:Tb 1.20 102 112 Comparative (Lead Foil/0.5 mm) Gd₂O₂S:Tb1.20 100 100 Example 9

As obvious from Table 3, each X-ray intensifying screen due toEmbodiments 5 through 10 shows the sensitivity comparative with those ofexisting fluorometallic screens (Comparative Example 9) that employ leadfoil. That is, these intensifying screens due to the above embodimentsare obvious to have performance enough to be applied practically. Inaddition, each sharpness thereof is remarkably improved compared withthat of Comparative Example 9.

INDUSTRIAL APPLICABILITY

First and second intensifying screens of the present invention, whilepreventing the lowering of sensitivity and sharpness from occurring, areimproved in granularity due to the phosphor layer of two-layer structurethat is easy in produce and less of restricting factors. Radiationreceptors and radiation inspection devices that employ such radiationintensifying screens of the present invention are particularly effectivewhen high sensitivity of radiography system is aimed. Even in suchsystems, excellent recognizability can be obtained.

Third intensifying screens of the present invention, while having theabsorption of high energy X-rays comparable with that of existingfluorometallic intensifying screens that employ lead foil, are improvedfurther in sensitivity, sharpness and granularity. Such intensifyingscreens can be employed effectively in X-ray radiography using highenergy X-rays and such radiation intensifying screens that can cope withhigh energy X-rays can be provided easily and less expensively.

What is claimed is:
 1. An intensifying screen, comprising: a support; afirst phosphor layer disposed on the support and constituted ofparticles of a first phosphor having an average particle diameter of D₁and a range coefficient k, which expresses particle size distribution,in the range of from 1.3 to 1.8; a second phosphor layer disposed on thefirst phosphor layer and constituted of particles of a second phosphorhaving an average particle diameter of D₂ that satisfies D₂>D₁ and arange coefficient k of particle size distribution of in the range offrom 1.5 to 2.0; and a protective film disposed on the second phosphorlayer.
 2. The intensifying screen as set forth in claim 1: whereinaverage particle diameter D₁ of the particles of the first phosphor isin the range of from 1 to 5 μm and average particle diameter D₂ of theparticles of the second phosphor is in the range of from 5 to 20 μm. 3.The intensifying screen as set forth in claim 1: wherein when a rangecoefficient of the particles of the first phosphor is k₁ and a rangecoefficient of the particles of the second phosphor is k₂, the particlesof the first and second phosphors satisfy a relationship of k₁<k₂. 4.The intensifying screen as set forth in claim 1: wherein when a coatingweight per unit area of the particles of the first phosphor in the firstphosphor layer is CW₁ and a coating weight per unit area of theparticles of the second phosphor in the second phosphor layer is CW₂,the ratio of the CW₁ and CW₂ (CW₁:CW₂) is in the range of from 8:2 to6:4.
 5. The intensifying screen as set forth in claim 1: wherein thefirst and second phosphors layer comprises rare earth phosphors.
 6. Aradiation receptor, comprising: a film for detecting radiation; a frontintensifying screen laminated along a surface of subject side of thefilm and consisting of the intensifying screen as set forth in claim 1;a back intensifying screen laminated along a surface of opposite sidefrom the subject side and consisting of the intensifying screen as setforth in claim 1; and a cassette accommodating the front intensifyingscreen, the film and the back intensifying screen.
 7. The radiationreceptor as set forth in claim 6: wherein when a total coating weightper unit area of phosphor particles of the first and second phosphorlayers of the front intensifying screen is TCW_(f) and a total coatingweight per unit area of phosphor particles of the first and secondphosphor layers of the back intensifying screen is TCW_(b), the ratio ofTCW_(f) and TCW_(b) (TCW_(f):TCW_(b)) is in the range of 3:7 to 4:6. 8.A device for radiation inspection, comprising: a radiation source; andthe radiation receptor that is set forth in claim 6 disposed opposite tothe radiation source through a subject.
 9. An intensifying screen,comprising: a support; a first phosphor layer disposed on the supportand constituted of particles of a first phosphor having an averageparticle diameter of D₁; a second phosphor layer disposed on the firstphosphor layer and constituted of particles of a second phosphor havingan average particle diameter of D₂ satisfying D₂>D₁; and a protectivefilm disposed on the second phosphor layer; wherein when a coatingweight per unit area of the particles of the first phosphor in the firstphosphor layer is CW₁ and a coating weight per unit area of theparticles of the second phosphor in the second phosphor layer is CW₂,the ratio of the CW₁ and CW₂ (CW₁:CW₂) is in the range of from 8:2 to6:4.
 10. The intensifying screen as set forth in claim 9: whereinaverage particle diameter D₁ of the particles of the first phosphor isin the range of from 1 to 5 μm and average particle diameter D₂ of theparticles of the second phosphor is in the range of from 5 to 20 μm. 11.The intensifying screen as set forth in claim 9: wherein the first andsecond phosphors layer comprises rare earth phosphors.
 12. A radiationreceptor, comprising: a film for detecting radiation; the frontintensifying screen that is laminated along on a surface of subject sideof the film and is set forth in claim 9; the back intensifying screenthat is laminated along on a surface opposite to the subject side of thefilm and is set forth in claim 9; and a cassette accommodating the frontintensifying screen, the film and the back intensifying screen.
 13. Theradiation receptor as set forth in claim 12: wherein when a totalcoating weight per unit area of phosphor particles of the first andsecond phosphor layers of the front intensifying screen is TCW_(f) and atotal coating weight per unit area of phosphor particles of the firstand second phosphor layers of the back intensifying screen is TCW_(b),the ratio of TCW_(f) and TCW_(b) (TCW_(f):TCW_(b)) is in the range of3:7 to 4:6.
 14. A device for radiation inspection, comprising: aradiation source; the radiation receptor that is set forth in claim 12disposed opposite to the radiation source through a subject.
 15. Anintensifying screen, comprising: a support; a phosphor layer disposed onthe support; a protective layer disposed on the phosphor layer; and apowder layer disposed between the support and the phosphor layer,consisting of at least one kind of particles selected from simpleparticles of metal, alloy particles consisting mainly of metal andcompound particles consisting mainly of metal, and having a thickness of2 to 40 kg/m² in terms of weight per unit area.
 16. The intensifyingscreen as set forth in claim 15: wherein the metal is heavy metal. 17.The intensifying screen as set forth in claim 16: wherein the heavymetal is at least one kind selected from W, Mo, Nb and Ta.
 18. Theintensifying screen as set forth in claim 16: wherein an amount of themetal in the particles constituting the powder layer is 60% or more byweight.
 19. The intensifying screen as set forth in claim 16: whereinthe particles are at least one kind selected from W—Re alloy, W—Moalloy, W—Nb alloy, W—Ta alloy, Mo—Nb alloy, Mo—Ta alloy, Nb—Ta alloy,WC, WO₃, MoO₃, MoC, Nb—C and Ta—C.
 20. The intensifying screen as setforth in claim 16: wherein the compounds consisting mainly of the metalare consisting of at least one kind selected from carbides of the metaland oxides of the metal.
 21. The intensifying screen as set forth inclaim 15: wherein the intensifying screen is one that is used withX-rays of high energy.